Glass-based pigments in painting: smalt blue and lead–tin yellow type II

Glass-based pigments have an important role in the panorama of artistic production due to the fact that their manufacturing processes involve a combination of different skills and understanding, and they have a role in disciplines ranging from glass technology to metallurgy, from glazed ceramic to stone imitation and from vitreous mosaic tesserae to painting materials. The main goal of this manuscript is to present a critical review of the literature relating to blue smalt and "yellow glass" (lead-tin yellow type II) pigments: presenting their historical background, the analytical protocol, the processes of alteration and decay and finally tracing issues. Several case studies analyzed by the authors will be presented. Particular attention was devoted to the correlation between micro-textural features and decay processes affecting the studied pigments, though the widespread heterogeneity of the analyzed materials and the variability of the artistic techniques in which the pigments were used as well as the effect of the relevant (micro-)environmental conditions dictate a cautious approach. These studies are presented in the context of information about the chain of production, the selection of the raw materials and relevant provenance studies.